Deployable antennas find use on board spacecraft as an element of a space borne radiometer, radar or communication systems. At RF frequencies and higher the form of that antenna typically includes a deployable dish shaped reflector or, as variously termed, parabolic reflector whose surface reflects microwave energy. The general design and principles of RF operation of parabolic reflectors and the antennas formed therewith are fairly well understood and aptly described in the technical literature.
To minimize storage requirements on board the spacecraft, the antenna's reflector is constructed to be deployable. That is, the reflector folds into a much smaller sized configuration for stowage for the spacecraft's launch. Thereafter, when orbit in outer space is achieved, the reflector is unfolded outside the spacecraft to cover a much larger area. To accomplish such deployability the reflector structure incorporates various mechanical devices and structure that accomplishes folding and unfolding. It also includes a light weight pliant reflective mesh material, which serves as the reflective surface.
Typically the deployable reflector is folded but once, and that folding is accomplished at the time of the reflector's manufacture. Once deployed, the reflector remains deployed throughout its operational life in space; there is no need for it to re-fold. Not only does the reflector's structure incorporate foldable joint structures, but, to minimize launch weight, those structural elements are as strong and light in weight as existing technology permits.
A number of different types of deployable reflectors for space borne application have appeared in the past, the newest of which is the perimeter truss reflector, an advanced design that allows reflective surfaces to cover areas of much larger size and offers the greatest benefit. An example of an early perimeter truss reflector design is found in U.S. Pat. No. 5,680,145 granted Oct. 21, 1997 to Thomson et al, assigned to Astro Aerospace Corp. Another such reflector, more relevant to the present invention, is the more advanced design presented in the application of Messrs. Gilger & Parker Ser. No. 09/080,767 filed May 18, 1998 and now U.S. Pat. No. 6,028,570, granted Feb. 22, 2000, assigned to the present assignee, which is incorporated herein by reference, and sometimes referred to herein as the Gilger & Parker reflector or truss. The present invention is applied to a deployable perimeter truss antenna of the type described in the Gilger & Parker patent application and may be adapted to other deployable reflectors as well.
The principal elements of the deployable perimeter truss design include the reflective surface, the perimeter truss, and a catenary system; the latter being a series of tension lines attached to the truss that shapes and supports the reflective surface to the parabolic shape. As unfolded and deployed, the perimeter truss reflector appears as a large diameter short hollow cylinder, with the dish-shaped reflective surface, supported by the catenary system, covering one end of that cylindrical structure. The truss's cylindrical wall comprises a skeletal frame of tubular members in a closed loop, that in appearance, in many respects, is reminiscent of the frame of a steel skyscraper, but with the top end of the skyscraper's frame wrapped around into a circle and joined to its bottom end.
The reflective surface is formed of pliant reflective material. That material may comprise a pliant metal gauze, mesh, cloth-like material or a thin metallized membrane, or any other material as well. At the higher RF frequencies the mesh material is formed of very fine gold plated filaments joined in a fine mesh that resembles women's nylon stocking and is almost invisible to the eye. At the lower RF frequencies the mesh may be more coarse in nature and resemble chicken coop wire.
To mold and shape as well as to hold the reflective mesh in place on the truss, typically, the front and rear ends of the truss contains a geodesic backup structure as found in the Thompson patent or a catenary system, the series of tension lines, catenaries, that structurally define the parabolic surface in a skeletal or wire form. The catenaries are supported at the trusses peripheral end edges and extend across the end of the truss.
The catenary lines located on the trusses front end overlie and are aligned with like catenary lines supported on the trusses rear end. By tying or otherwise connecting various points along a single catenary to like points on the underlying catenary line with ties of different selected lengths, referred to as drop lines, each catenary may be shaped to approximate a portion of a parabolic curve. By judiciously shaping each catenary in the series to an appropriate portion of a parabolic curve, an entire parabolic surface is skeletally defined. That skeletal paraboloid surface serves as a wall, seat or bed, however characterized, on which the reflective surface is placed, somewhat like a bed sheet laid upon a bed, or, alternatively, as a tissue blown against a window screen.
As folded up for stowage, the reflector appears as an elongate cylindrical shape formed of a collection of structural elements closely packed together, often referred to as a "barrel". The reflective mesh material is packed inside that barrel.
The Gilger & Parker perimeter truss reflector, earlier referred to, is a new design. For a given diameter as deployed, that unique reflector folds to a more compact size than prior perimeter truss designs. As a consequence for a given application, reflectors of the Gilger & Parker design may fit within the available storage space on some rockets, when reflectors constructed in accordance with prior older designs could not. That advantage, for one, allows a mission to be accomplished without requiring a new larger rocket to first be designed and built.
The Gilger & Parker perimeter truss incorporates a series of deployable spars which, as deployed, extend outwardly from the front and rear ends of a truss that is formed of structural members. An outer end of each of the spars is connected to an associated tension line that forms a hoop about the respective end of the reflector. Those ends also attach to a respective catenary line, the latter line supported from the end of those spars. The deployable spars give the truss a greater expanse. Together with the hoop tension lines the deployable spar arrangement avoids any necessity for using stiff structural members for the interconnection, avoiding the greater weight inherent in structural members. For a given deployed diameter, the Gilger & Parker reflector is thus lower in weight than the prior designs. There are other advantages not here described for which the interested reader is referred to the cited Parker & Gilger patent application.
The foregoing structure, only briefly summarized, may be difficult for the lay person to visualize, at least initially. Some such readers might find it helpful to briefly refer to some of the partial illustrations of the Gilger and Parker perimeter truss reflector presented in the first two drawing figures and/or make reference to the cited patents or applications before proceeding further in this description.
Unfortunately, the smaller stowed size of the Gilger & Parker perimeter truss reflector has an inherent drawback. Space deployable parabolic mesh reflectors require very elaborate and complex mesh stowage systems. Generally the mesh material is susceptible to damage from tight fold lines; and the mesh could possibly snag or get caught on many structural pieces of the truss. To avoid those potential inherent problems, the stowage systems employed in the past generally fold the mesh inside the "barrel" formed by the truss's folding ribs. With the advent of the new deployable perimeter truss reflector presented in the cited application to Gilger and Parker, the available interior space for storing the mesh is considerably reduced.
The available stowage volume in the Gilger &Parker reflector appears marginal for existing mesh folding techniques. To successfully pack the mesh using existing techniques is time consuming, tedious and difficult and requires the time and attention of many assembly technicians. Unless a suitable mesh structure and folding procedure is available the great advantages resulting from use of that novel reflector design might not be realized.
Accordingly, an object of the invention is to provide a more efficient method of packing the truss reflector's mesh and catenary system for stowage.
Another more specific object of the invention is to provide a method to pack the reflective mesh of a Gilger and Parker deployable spar type perimeter truss reflector.
A further object of the invention is to pack the reflective mesh and catenary lines of a foldable perimeter truss reflector into a compact small sized package that conveniently fits within the truss's barrel configuration as stowed.
An additional object of the invention is to provide a modification to the catenary support system that accommodates and enables more efficient mesh packing.
And a still additional object of the invention is to provide a new tool with which the new method of packing the truss reflector's mesh may be readily practiced.